1. Field of the Invention
The present invention relates to a method of hardening gelatin, more particularly, it relates to a method of hardening gelatin used for silver halide photographic materials.
2. Description of the Prior Art
Gelatin is used as binder for many photographic materials. For example, gelatin is used as the main constituent of a silver halide sensitive emulsion layer, emulsion protective layer, filter layer, intermediate layer, anti-halation layer, backing layer, film base subbing layer and baryta layer.
These photographic materials containing gelatin are treated with various aqueous solutions having different pH's and temperatures. Since a layer containing gelatin not treated with a hardening agent has gelatin-like properties, it has poor water resistance and swells excessively in an aqueous solution, so that the mechanical strength is greatly lowered, and, in an extreme case, the gelatin layer is sometimes dissolved, particularly when in an aqueous solution having a high temperature of above 30.degree. C. or a highly alkaline aqueous solution.
These properties are fatal defects as physical properties of photographic material layers.
In order to raise the water resistance, heat resistance and scratch resistance of a gelatin layer by hardening, many compounds are known which are used in the production of photographic materials. Examples are formaldehyde, glutaraldehyde and like aldehyde type compounds; compounds having a reactive halogen as described in U.S. Pat. No. 3,288,775 and so on; compounds containing a reactive ethylenic unsaturated bonding as described in U.S. Pat. No. 3,635,718 and so on; aziridine type compounds as described in U.S. Pat. No. 3,017,280; epoxy compounds as described in U.S. Pat. No. 3,091,537; and halogenocarboxyaldehydes such as mucochloric acid, dioxanes such as dihydroxydioxane and dichlorodioxane, or inorganic hardeners such as chromium alum and zirconium sulfate.
However, most of these hardeners have serious defects with regard to photographic properties, the gelatin hardening velocity (film hardening velocity) is not always sufficiently fast, so that film hardening slowly proceeds with the passage of time after the production of the photographic materials (i.e., after-hardening), the photographic properties (e.g., sensitivity and color balance) which are affected by the state of gelatin film hardening are not stabilized and the properties immediately after production vary slowly with the passage of time.
On the other hand, certain kinds of hardeners which have a remarkably high hardening velocity (to thereby prevent after hardening) are known and are called rapid hardeners.
However, most rapid hardeners lead to serious defects in photographic material production steps that a hardening reaction proceeds before film formation (e.g., in a tank or in a coating head for coating before coating of the photographic emulsion on a support) when the rapid hardener is added to a gelatin solution so that the viscosity of the gelatin solution is raised and coating of the photographic emulsion becomes very difficult.